A power steering apparatus is widely used as an apparatus for making it possible to reduce the power required for an operator to control the steering wheel when applying an angle to the steered wheels (normally the front wheel except in the case of special vehicles such as a fork lift). Also, in recent years, electric-powered power steering apparatuses that use an auxiliary power source are beginning to become popular as this kind of power steering apparatus. Compared to a hydraulic power steering apparatus, an electric-powered power steering apparatus has advantages in that it can be made more compact and lighter, that the size (torque) of the auxiliary power is easy to control and that there is less power loss of the engine. FIG. 40 shows the basic construction of a conventional electric-powered power steering apparatus.
In the middle of the steering shaft 2, which is turned by operating the steering wheel 1, there are a torque sensor 3 that detects the direction and size of the torque applied to the steering shaft 2 from the steering wheel 1, and a speed reducer 4. The output side of this speed reducer 4 connects to the middle of the steering shaft 2, and similarly, the input side connects to the rotating shaft of an electric motor 5. Also, the detection signal from the torque sensor 3 is, together with a signal indicating vehicle speed, input to a controller 6 for controlling the electric power to the electric motor 5. Moreover, in the past, a worm-gear reducer having a large lead angle and reversibility in the power transmission direction has been generally used as the speed reducer 4. In other words, the worm wheel that receives the torque is fastened to the middle of the steering shaft 2, and the worm on the worm shaft, which is connected to and fastened to the rotating shaft of the electric motor 5 that applies the torque, meshes with the worm wheel.
When the steering wheel 1 is operated and the steering shaft 2 is turned in order to apply an angle to the steered wheels 14, the torque sensor 3 detects the turning direction and torque of the steering shaft 2, and sends a signal indicating the detected values to the controller 6. The controller 6 then sends electric power to the electric motor 5 and by way of the speed reducer 4 turns the steering shaft 2 in the same direction as the direction of rotation of the steering wheel 1. As a result, the tip end of the steering shaft 2 (bottom end in FIG. 40) turns by a torque larger than the torque based on the force applied from the steering wheel 1.
The rotation of the tip end of the steering shaft 2 is transmitted to the input shaft 10 of the steering gear 9 by way of the universal joints 7 and intermediate shaft 8. This input shaft 10 turns the pinion 11 of the steering gear 9, and pushes or pulls the tie rod 13 by way of the rack 12 to apply the desired angle to the steered wheels 14. As can be clearly seen from the explanation above, the torque that is transmitted to the intermediate shaft 8 from the tip end of the steering shaft 2 by way of the universal joints 7 is larger than the torque applied to the base end of the steering shaft 2 (top end in FIG. 40) from the steering wheel 1 by the amount of the auxiliary power supplied from the electric motor by way of the speed reducer 4. Therefore, the force required by the operator in order to operate the steering wheel 1 and to apply an angle to the steered wheels 14 becomes smaller by the amount of auxiliary power. Also, disclosures of prior technology related to the invention are given below.
(1) International Disclosure Pamphlet 99/65758, (2) International Disclosure Pamphlet 99/11502, (3) JP Patent Publication Tokukai Hei 3-112784, (4) JP Utility Model Publication Jitsukai Sho 63-46281, (5) JP Patent Publication Tokukai Hei 11-43062, (6) International Disclosure Pamphlet 02/32741, (7) JP Patent Publication Tokukai 2002-98198, (8) JP Patent Publication Tokukai Hei 10-281235, (9) JP Patent Publication Tokukai 2000-43739, (10) JP Patent Publication Tokukai 2002-21943, (11) JP Patent Publication Tokukai 2002-29434, (12) JP Patent Publication Tokukai Hei 9-30432, (13) JP Patent Publication Tokukai 2001-233224, (14) JP Patent Publication Tokukai 2001-233225
In the case of the conventionally used electric-powered power steering apparatus described above, a worm-gear reducer is used as the speed reducer 4 in between the electric motor 5 and the steering shaft 2. However, there is unavoidable backlash in this worm-gear reducer. This backlash becomes larger as the errors in dimensions and assembly errors in the components of the worm-gear reducer such as the worm shaft, worm wheel and bearing for supporting these members becomes larger. When there is large backlash like this, there is large collision between some of the teeth surfaces and the worm wheel and worm gear, which can cause loud, disturbing noise.
It is considered possible to reduce this backlash by suitably fitting the components of the worm-gear reducer together and by taking the precision of dimensions into consideration. However, when reducing this backlash, the work of controlling the precision of the dimensions and assembling the components becomes troublesome, and causes the cost to increase.